This invention relates generally to methods and apparatus for CT imaging and other radiation imaging systems, and more particularly to reducing aliasing artifacts.
In at least some computed tomography (CT) imaging system configurations, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as an xe2x80x9cimaging planexe2x80x9d. The x-ray beam passes through an object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at a detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged, so the angle at which the x-ray beam intersects the object constantly changes. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal spot. X-ray detectors typically include a collimator for collimating x-ray beams received at the detector, a scintillator adjacent the collimator, and photodetectors adjacent to the scintillator.
The scan speed of CT systems has nearly quadrupled in recent years. Increasing scan speed results in improved temporal resolution and reducing the amount of time required to perform each scan. Therefore, patient scans are completed more quickly and more scans can be performed.
Increased scan speeds, however, can result in image artifacts. For example, to avoid view aliasing artifacts, the data-sampling rate needs to be increased proportionally with the scan speed. For a 1.0 s scan speed, i.e., 1 second per revolution, a data acquisition system (DAS) should sample signals from the detector at 984 Hz to avoid aliasing artifacts. For a 0.8 s scan speed, the DAS sample rate should be 1230 Hz to avoid aliasing artifacts. When the scan speed reaches 0.5 s per revolution, the DAS sampling rate should be 1968 Hz to avoid aliasing artifacts. Conventional DAS, however, sample at a rate of 1408 Hz.
Adaptive view interpolation can be utilized to eliminate or reduce aliasing artifacts. Although adaptive view interpolation is effective in most clinical conditions, the interpolation is performed on every scan regardless of the object being scanned. Since the interpolation is performed on all scans, the relative contribution of the interpolated views is kept to a moderate level to avoid significant degradation of spatial resolution. As a result, some residual aliasing artifacts may be present. Also, although aliasing is predominately in a horizontal direction, aliasing may occur in other directions. For example, if a patient is scanned lying on his or her side, aliasing may occur in a vertical direction.
Methods and apparatus for reducing aliasing artifacts generated as a result of performing high speed scans are described. In an exemplary embodiment of the method, view aliasing artifacts are reduced by generating an aliasing index for each view, and the aliasing index is used to determine the region and the amount of synthesized views required to reduce the aliasing artifact. The aliasing index is generated from the projection data.
The above described method for reducing, if not eliminating, aliasing artifacts facilitates performing high speed scans even if the DAS sample rate is not fast enough to avoid such artifacts. Further, implementing the method does not require that additional hardware be used or replaced.